What is the reason to make quad or triangle faces on a surface?
There are several reasons to use only quads in your models.


some modelling tools only work with quads. eg Loop cut will cut quads but stop at a triangle or ngon.
triangles and ngons can deform badly when animated.
ngons are automatically broken into triangles to be drawn to screen, particularly on non flat ngons, these may not be cut in ideal locations causing noticeable shading artefacts.
the subsurface modifier also doesn't subdivide ngons very well.
generated textures can also suffer from bad co-ordinate generation on ngons.


For flat surfaces shaded in a single colour you can get away with leaving ngons in your final model if you want to.

To get quads you need to cut your mesh up where you want the new edges. you can select two vertices and press J to cat a new edge between them or you can use the knife tool K to cut new edges and vertices where you want them.

An easy way that can work under some conditions is to use the remesh modifier. Once you are happy with the settings you can apply the modifier to make the new mesh permanent.